1. Field of the Invention
The present invention relates to coating techniques as applied in particular to the protection of the so-called organic glasses which are made of plastic.
More specifically, the invention is concerned with a method of preparation of a coating composition based on polysiloxane resin an extends to the use of the composition obtained as a varnish for the antiabrasion protection of polycarbonate substrates. The essential aim of the invention is to increase the refractive index of varnishes of this type and accordingly finds a preferred field of application in ophthalmic optics.
Nevertheless, the invention is not limited to this particular application or to the protection of polycarbonates. It may also prove useful in all cases, for example, in which a transparent substrate requires a varnish having a high refractive index.
2. Description of the Prior Art
Plastics in general, and more especially those employed in the manufacture of ophthalmic lenses, usually have poor mechanical surface characteristics with in particular low resistance to abrasion and not-negligible sensitivity to aggressive solvents. It is therefore a common practice to protect them with an antiabrasion coating. Materials based on polycarbonates do not escape from this requirement. At the same time, it is known that the advantage offered by the use of polycarbonates in the manufacture of ophthalmic lenses lies mainly in the high refractive index which they make it possible to obtain with values of the order of 1.5 to 1.6, for example. But, it becomes difficult in this case to protect them with a suitable coating having a refractive index which must be sufficiently close to that of the lenses to avoid giving rise to interference phenomena which would impair the attractiveness of their appearance and their optical qualities.
In point of fact, the coating compositions employed up to the present time in this field produce maximum refractive indices of the order of 1.49 to 1.50. These compositions are usually based on polymerizable organic silanes and colloidal silica, and they are contemplated in particular for coating substrates of polycarbonate of bis-allyl diethylene-glycol. The preparation of these compositions as described for example in U.S. Pat. No. 3,986,997 involves a stage of hydrolysis of polyfunctional alkoxysilanes or epoxysilanes, thus giving rise to reactive silanol groups which condense and form polysiloxanes.
In order to improve these compositions, it has been proposed to introduce organic compounds of titanium and in particular titanium esters. Indeed it has been known for a long time (see French patent 1,473,183) that coating compositions may be improved through using the product from the reaction of an organic titanate, typically tetraisopropyl titanate with an epoxy-silane such as GLYMO (glyceryloxypropyl tri-methoxysilane). But, this solution involves considerable difficulties of preparation when using hydrolyzed alkoxysilanes, due to the reactivity of the titanates employed with respect to the organic silanes and to their tendency to undergo degradation in the presence of water and lead to the formation of tetravalent titanium hydroxide, which is insoluble.
French patent No. 2,446,308 proposes to hydrolyze completely an alkyl titanate in a water-alcohol mixture in order to obtain a salt of colloidal titanium dioxide which is then mixed with a hydrolyzate of silanes. The composition obtained is formed of colloidal silica and of colloidal titanium dioxide dispersed in a solution of siloxanol in a system of water-alcohol co-solvents. The disadvantage of such a method lies in the relative instability exhibited by titanium dioxide sols when they are added to the hydrolyzates of silanes, with the result that, as the concentration of titanium dioxide increases, so it becomes more difficult to obtain a satisfactory dispersion of the particles of titanium dioxide in the final composition without formation of precipitate.
Moreover, German patent No. 3,407,087 describes the preparation of a varnish from a mixture of alkoxysilanes and of derivatives of titanium such as ethyl titanate and isopropyl titanate. Hydrolysis of this mixture is effected in a two-step process. There is first carried out a precondensation in the presence of a quantity of water which is smaller than that required for stoichiometric hydrolysis of the hydrolyzable groups which are present. There is then carried out a complementary condensation with a quantity of water at least equal to that which corresponds to stoichiometric hydrolysis of the remaining hydrolyzable groups. This technique results in the formation of precipitates during preparation of the coating composition. This entails the need for filtrations and consequently causes losses of materials, with the result that the characteristics of the coating thus obtained are uncertain.
European patent Application No. 0263428 describes a similar technique by explaining at greater length the condensation and prepolymerization reactions which take place when preparing an alkoxysilane hydrolyzate which is partially hydrolyzed so that it still contains active silanol groups, to which is then added an alkyl titanate such as tetraethoxy-titanium, before adding water until complete hydrolysis of the hydrolyzable groups, there being thus obtained a coating composition which is hardenable by means of complementary condensation reactions within the prepolymer contained therein.
Experience shows that, here again, the precautions to be taken in order to avoid the formation of precipitates of titanium dioxide TiO.sub.2 resulting from hydrolysis of alkoxysilane are incompatible with the practice of industrial manufacture and that, in addition, it is impossible to obtain a coating composition which has a sufficiently long "pot life" or stability in time prior to application on a substrate.
And in each of the methods disclosed in the prior art, it has proved impossible to prepare useful coating compositions with the concentrations of titanium dioxide which would have been necessary in order to increase the refractive index of the final varnish in a desirable manner.
In order to solve these difficulties, the invention proposes to employ special organic compounds of titanium consisting of titanium esters of carboxylic acids, i.e. acyl titanates, and to use them instead of the titanates of the prior art while avoiding their hydrolysis prior to reaction with the organic silane.